Source: Mr. Bridge
As a defender, switching to the right suit is a wonderful habit to get into. Your aim should always be to lead through strength and up to weakness. Generally, it is dummy that will tell you what to lead, not your own hand or your partner. When dummy is on your left, you are leading through it, which means you are looking to lead through strength.
You lead the 
A, but your partner plays the 4 and dummy has club length, so you choose to switch, but which suit do you choose? Dummy is on your left so you are looking to lead through a stronger suit. Getting used to this tactic is so important. Intuitively, you might feel you want to lead dummy’s weaker suits, but this plays into declarer’s hand: allowing his strength to dominate your partner’s. Instead, you follow the tip and lead a diamond, 
6. Your partner ducks, but plays the 8 to suggest he likes your switch.
Declarer has little chance: he can take four spade tricks but your accurate switch should keep him to seven tricks. See how badly a heart switch would fare: exposing your partner’s high cards before declarer’s and allowing an extra heart trick. Here is a similar layout:
Once again, you lead the A and are disappointed to see dummy. Choosing to switch, the difference this time is the presence of the 
A. However, when you are looking to lead through strength, it is important to understand why: you want your partner to be able to kill the strength, so it is important that you are leading through ‘beatable’ strength. An ace is unbeatable and, therefore, not good to lead through. You should ignore bare aces when considering leading through strength, so once again, you switch to diamonds. Declarer falls short again, most likely taking a heart finesse, and playing a club, allowing West to win and play a second diamond to defeat the contract with two clubs and three diamonds. Knowing which suit to switch to is such an important aspect of defence. Almost always, your choice of switch will be dictated by dummy, not by your hand or by a signal.
Focus on dummy and your defence will improve: when dummy is on your left, lead through heft.